Use separate channels for guest / host messages from sidebar

Use separate channels for sending messages to guests vs the host in the
sidebar, even for the common case when there is only one guest and it is
the same frame as the host.

This change makes it clear for readers which part of the annotator code
is intended to handle a particular message from the sidebar. It is also
a step towards supporting host frames that are not guests. This will be
needed in ebook readers where the host frame provides the navigation UI
and contains the frame displaying the book content, but should not be
annotatble itself.

 - Remove the `bridge` service in the sidebar. The `frameSync` service
   now provides the entry point for other services/components to make RPC calls
   to the host or guest frames. Currently the only use case is
   sending notifications to the host via `FrameSyncService.notifyHost`.

 - Create separate `Bridge` instances in `FrameSyncService` for sidebar
   <-> guest and sidebar <-> host communication. The sidebar <-> guest
   bridge works the same as before. The sidebar <-> host bridge
   is established by having `FrameSyncService` create a MessageChannel
   when sending the `hypothesisSidebarReady` notification to the host.

   The sidebar / host frames then add respective ports of this channel
   to a Bridge instance.

 - Change the various existing RPC calls between frames to use either
   the guest <-> sidebar or host <-> sidebar communication channels as
   appropriate

src/annotator/sidebar.js

 import Hammer from 'hammerjs';
 
+import { Bridge } from '../shared/bridge';
 import events from '../shared/bridge-events';
 import { ListenerCollection } from '../shared/listener-collection';
 
@@ -63,6 +64,8 @@ export default class Sidebar {
   constructor(element, eventBus, guest, config = {}) {
     this._emitter = eventBus.createEmitter();
 
+    this._sidebarRPC = new Bridge();
+
     /**
      * The `<iframe>` element containing the sidebar application.
      */
@@ -199,8 +202,9 @@ export default class Sidebar {
      */
     this.ready = new Promise(resolve => {
       this._listeners.add(window, 'message', event => {
-        const data = /** @type {MessageEvent} */ (event).data;
-        if (data?.type === 'hypothesisSidebarReady') {
+        const messageEvent = /** @type {MessageEvent} */ (event);
+        if (messageEvent.data?.type === 'hypothesisSidebarReady') {
+          this._sidebarRPC.createChannel(messageEvent.ports[0]);
           resolve();
         }
       });
@@ -220,22 +224,19 @@ export default class Sidebar {
   }
 
   _setupSidebarEvents() {
-    annotationCounts(document.body, this.guest.crossframe);
+    annotationCounts(document.body, this._sidebarRPC);
     sidebarTrigger(document.body, () => this.open());
-    features.init(this.guest.crossframe);
+    features.init(this._sidebarRPC);
 
-    this.guest.crossframe.on('openSidebar', () => this.open());
-    this.guest.crossframe.on('closeSidebar', () => this.close());
+    this._sidebarRPC.on('openSidebar', () => this.open());
+    this._sidebarRPC.on('closeSidebar', () => this.close());
 
     // Sidebar listens to the `openNotebook` event coming from the sidebar's
     // iframe and re-publishes it via the emitter to the Notebook
-    this.guest.crossframe.on(
-      'openNotebook',
-      (/** @type {string} */ groupId) => {
+    this._sidebarRPC.on('openNotebook', (/** @type {string} */ groupId) => {
       this.hide();
       this._emitter.publish('openNotebook', groupId);
-      }
-    );
+    });
     this._emitter.subscribe('closeNotebook', () => {
       this.show();
     });
@@ -249,7 +250,7 @@ export default class Sidebar {
     ];
     eventHandlers.forEach(([event, handler]) => {
       if (handler) {
-        this.guest.crossframe.on(event, () => handler());
+        this._sidebarRPC.on(event, () => handler());
       }
     });
   }
@@ -427,7 +428,7 @@ export default class Sidebar {
   }
 
   open() {
-    this.guest.crossframe.call('sidebarOpened');
+    this._sidebarRPC.call('sidebarOpened');
     this._emitter.publish('sidebarOpened');
 
     if (this.iframeContainer) {
@@ -466,7 +467,7 @@ export default class Sidebar {
    * @param {boolean} shouldShowHighlights
    */
   setAllVisibleHighlights(shouldShowHighlights) {
-    this.guest.crossframe.call('setVisibleHighlights', shouldShowHighlights);
+    this._sidebarRPC.call('setVisibleHighlights', shouldShowHighlights);
   }
 
   /**

src/annotator/test/sidebar-test.js

@@ -10,7 +10,6 @@ const EXTERNAL_CONTAINER_SELECTOR = 'test-external-container';
 
 describe('Sidebar', () => {
   const sandbox = sinon.createSandbox();
-  let fakeCrossFrame;
   let fakeGuest;
 
   // Containers and Sidebar instances created by current test.
@@ -23,6 +22,8 @@ describe('Sidebar', () => {
   let FakeToolbarController;
   let fakeToolbar;
 
+  let fakeBridge;
+
   before(() => {
     sinon.stub(window, 'requestAnimationFrame').yields();
   });
@@ -65,17 +66,12 @@ describe('Sidebar', () => {
   beforeEach(() => {
     sidebars = [];
     containers = [];
-    fakeCrossFrame = {
-      on: sandbox.stub(),
-      call: sandbox.stub(),
-    };
 
     class FakeGuest {
       constructor() {
         this.element = document.createElement('div');
         this.contentContainer = sinon.stub().returns(document.body);
         this.createAnnotation = sinon.stub();
-        this.crossframe = fakeCrossFrame;
         this.fitSideBySide = sinon.stub();
         this.setVisibleHighlights = sinon.stub();
       }
@@ -100,7 +96,14 @@ describe('Sidebar', () => {
 
     sidebars = [];
 
+    fakeBridge = {
+      call: sinon.stub(),
+      createChannel: sinon.stub(),
+      on: sinon.stub(),
+    };
+
     $imports.$mock({
+      '../shared/bridge': { Bridge: sinon.stub().returns(fakeBridge) },
       './toolbar': {
         ToolbarController: FakeToolbarController,
       },
@@ -155,6 +158,27 @@ describe('Sidebar', () => {
     });
   });
 
+  function sendSidebarReadyMessage() {
+    const channel = new MessageChannel();
+    const event = new MessageEvent(
+      'message',
+      {
+        data: { type: 'hypothesisSidebarReady' },
+      },
+      [channel.port1]
+    );
+
+    window.dispatchEvent(event);
+
+    return event;
+  }
+
+  it('creates Bridge for host <-> sidebar RPC calls when `hypothesisSidebarReady` message is received', () => {
+    createSidebar();
+    const event = sendSidebarReadyMessage();
+    assert.calledWith(fakeBridge.createChannel, event.ports[0]);
+  });
+
   describe('#ready', () => {
     it('returns a promise that resolves when `hypothesisSidebarReady` message is received', async () => {
       const sidebar = createSidebar();
@@ -166,11 +190,7 @@ describe('Sidebar', () => {
         'not-ready'
       );
 
-      window.dispatchEvent(
-        new MessageEvent('message', {
-          data: { type: 'hypothesisSidebarReady' },
-        })
-      );
+      sendSidebarReadyMessage();
 
       return sidebar.ready;
     });
@@ -281,10 +301,10 @@ describe('Sidebar', () => {
     });
   });
 
-  describe('crossframe listeners', () => {
+  describe('sidebar RPC call handlers', () => {
     const emitEvent = (event, ...args) => {
       const result = [];
-      for (let [evt, fn] of fakeCrossFrame.on.args) {
+      for (let [evt, fn] of fakeBridge.on.args) {
         if (event === evt) {
           result.push(fn(...args));
         }
@@ -308,7 +328,7 @@ describe('Sidebar', () => {
         assert.called(target);
       }));
 
-    describe('on "openNotebook" crossframe event', () => {
+    describe('on "openNotebook" event', () => {
       it('hides the sidebar', () => {
         const sidebar = createSidebar();
         sinon.stub(sidebar, 'hide').callThrough();
@@ -605,10 +625,10 @@ describe('Sidebar', () => {
   });
 
   describe('#setAllVisibleHighlights', () =>
-    it('sets the state through crossframe and emits', () => {
+    it('requests sidebar to set highlight visibility in guest frames', () => {
       const sidebar = createSidebar();
       sidebar.setAllVisibleHighlights(true);
-      assert.calledWith(fakeCrossFrame.call, 'setVisibleHighlights', true);
+      assert.calledWith(fakeBridge.call, 'setVisibleHighlights', true);
     }));
 
   it('hides toolbar controls when using the "clean" theme', () => {

src/sidebar/components/HypothesisApp.js

@@ -23,7 +23,6 @@ import TopBar from './TopBar';
 /**
  * @typedef {import('../../types/api').Profile} Profile
  * @typedef {import('../../types/config').MergedConfig} MergedConfig
- * @typedef {import('../../shared/bridge').Bridge} Bridge
  * @typedef {import('./UserMenu').AuthState} AuthState
  */
 
@@ -54,7 +53,7 @@ function authStateFromProfile(profile) {
 /**
  * @typedef HypothesisAppProps
  * @prop {import('../services/auth').AuthService} auth
- * @prop {Bridge} bridge
+ * @prop {import('../services/frame-sync').FrameSyncService} frameSync
  * @prop {MergedConfig} settings
  * @prop {import('../services/session').SessionService} session
  * @prop {import('../services/toast-messenger').ToastMessengerService} toastMessenger
@@ -68,7 +67,7 @@ function authStateFromProfile(profile) {
  *
  * @param {HypothesisAppProps} props
  */
-function HypothesisApp({ auth, bridge, settings, session, toastMessenger }) {
+function HypothesisApp({ auth, frameSync, settings, session, toastMessenger }) {
   const store = useStoreProxy();
   const hasFetchedProfile = store.hasFetchedProfile();
   const profile = store.profile();
@@ -99,7 +98,7 @@ function HypothesisApp({ auth, bridge, settings, session, toastMessenger }) {
   const login = async () => {
     if (serviceConfig(settings)) {
       // Let the host page handle the login request
-      bridge.call(bridgeEvents.LOGIN_REQUESTED);
+      frameSync.notifyHost(bridgeEvents.LOGIN_REQUESTED);
       return;
     }
 
@@ -117,7 +116,7 @@ function HypothesisApp({ auth, bridge, settings, session, toastMessenger }) {
   const signUp = () => {
     if (serviceConfig(settings)) {
       // Let the host page handle the signup request
-      bridge.call(bridgeEvents.SIGNUP_REQUESTED);
+      frameSync.notifyHost(bridgeEvents.SIGNUP_REQUESTED);
       return;
     }
     window.open(store.getLink('signup'));
@@ -157,7 +156,7 @@ function HypothesisApp({ auth, bridge, settings, session, toastMessenger }) {
     store.discardAllDrafts();
 
     if (serviceConfig(settings)) {
-      bridge.call(bridgeEvents.LOGOUT_REQUESTED);
+      frameSync.notifyHost(bridgeEvents.LOGOUT_REQUESTED);
       return;
     }
 
@@ -203,7 +202,7 @@ function HypothesisApp({ auth, bridge, settings, session, toastMessenger }) {
 
 export default withServices(HypothesisApp, [
   'auth',
-  'bridge',
+  'frameSync',
   'session',
   'settings',
   'toastMessenger',

src/sidebar/components/TopBar.js

@@ -16,13 +16,12 @@ import UserMenu from './UserMenu';
 /**
  * @typedef {import('../../types/config').MergedConfig} MergedConfig
  * @typedef {import('../components/UserMenu').AuthState} AuthState
- * @typedef {import('../../shared/bridge').Bridge} Bridge
  */
 
 /**
  * @typedef TopBarProps
  * @prop {AuthState} auth
- * @prop {Bridge} bridge
+ * @prop {import('../services/frame-sync').FrameSyncService} frameSync
  * @prop {boolean} isSidebar - Flag indicating whether the app is the sidebar or a top-level page.
  * @prop {() => any} onLogin - Callback invoked when user clicks "Login" button.
  * @prop {() => any} onLogout - Callback invoked when user clicks "Logout" action in account menu.
@@ -39,7 +38,7 @@ import UserMenu from './UserMenu';
  */
 function TopBar({
   auth,
-  bridge,
+  frameSync,
   isSidebar,
   onLogin,
   onLogout,
@@ -72,7 +71,7 @@ function TopBar({
   const requestHelp = () => {
     const service = serviceConfig(settings);
     if (service && service.onHelpRequestProvided) {
-      bridge.call(bridgeEvents.HELP_REQUESTED);
+      frameSync.notifyHost(bridgeEvents.HELP_REQUESTED);
     } else {
       store.toggleSidebarPanel('help');
     }
@@ -171,4 +170,4 @@ function TopBar({
   );
 }
 
-export default withServices(TopBar, ['bridge', 'settings', 'streamer']);
+export default withServices(TopBar, ['frameSync', 'settings', 'streamer']);

src/sidebar/components/UserMenu.js

@@ -28,7 +28,7 @@ import MenuSection from './MenuSection';
  * @typedef UserMenuProps
  * @prop {AuthStateLoggedIn} auth - object representing authenticated user and auth status
  * @prop {() => any} onLogout - onClick callback for the "log out" button
- * @prop {object} bridge
+ * @prop {import('../services/frame-sync').FrameSyncService} frameSync
  * @prop {MergedConfig} settings
  */
 
@@ -40,7 +40,7 @@ import MenuSection from './MenuSection';
  *
  * @param {UserMenuProps} props
  */
-function UserMenu({ auth, bridge, onLogout, settings }) {
+function UserMenu({ auth, frameSync, onLogout, settings }) {
   const store = useStoreProxy();
   const defaultAuthority = store.defaultAuthority();
 
@@ -57,7 +57,7 @@ function UserMenu({ auth, bridge, onLogout, settings }) {
     !isThirdParty || serviceSupports('onLogoutRequestProvided');
 
   const onSelectNotebook = () => {
-    bridge.call('openNotebook', store.focusedGroupId());
+    frameSync.notifyHost('openNotebook', store.focusedGroupId());
   };
 
   // Temporary access to the Notebook without feature flag:
@@ -70,7 +70,7 @@ function UserMenu({ auth, bridge, onLogout, settings }) {
   };
 
   const onProfileSelected = () =>
-    isThirdParty && bridge.call(bridgeEvents.PROFILE_REQUESTED);
+    isThirdParty && frameSync.notifyHost(bridgeEvents.PROFILE_REQUESTED);
 
   // Generate dynamic props for the profile <MenuItem> component
   const profileItemProps = (() => {
@@ -129,4 +129,4 @@ function UserMenu({ auth, bridge, onLogout, settings }) {
   );
 }
 
-export default withServices(UserMenu, ['bridge', 'settings']);
+export default withServices(UserMenu, ['frameSync', 'settings']);

src/sidebar/components/test/HypothesisApp-test.js

@@ -9,7 +9,7 @@ describe('HypothesisApp', () => {
   let fakeApplyTheme;
   let fakeStore = null;
   let fakeAuth = null;
-  let fakeBridge = null;
+  let fakeFrameSync;
   let fakeConfirm;
   let fakeServiceConfig = null;
   let fakeSession = null;
@@ -21,7 +21,7 @@ describe('HypothesisApp', () => {
     return mount(
       <HypothesisApp
         auth={fakeAuth}
-        bridge={fakeBridge}
+        frameSync={fakeFrameSync}
         settings={fakeSettings}
         session={fakeSession}
         toastMessenger={fakeToastMessenger}
@@ -67,8 +67,8 @@ describe('HypothesisApp', () => {
 
     fakeSettings = {};
 
-    fakeBridge = {
-      call: sinon.stub(),
+    fakeFrameSync = {
+      notifyHost: sinon.stub(),
     };
 
     fakeToastMessenger = {
@@ -228,7 +228,10 @@ describe('HypothesisApp', () => {
       it('sends SIGNUP_REQUESTED event', () => {
         const wrapper = createComponent();
         clickSignUp(wrapper);
-        assert.calledWith(fakeBridge.call, bridgeEvents.SIGNUP_REQUESTED);
+        assert.calledWith(
+          fakeFrameSync.notifyHost,
+          bridgeEvents.SIGNUP_REQUESTED
+        );
       });
 
       it('does not open a URL directly', () => {
@@ -299,9 +302,9 @@ describe('HypothesisApp', () => {
       const wrapper = createComponent();
       await clickLogIn(wrapper);
 
-      assert.equal(fakeBridge.call.callCount, 1);
+      assert.equal(fakeFrameSync.notifyHost.callCount, 1);
       assert.isTrue(
-        fakeBridge.call.calledWithExactly(bridgeEvents.LOGIN_REQUESTED)
+        fakeFrameSync.notifyHost.calledWithExactly(bridgeEvents.LOGIN_REQUESTED)
       );
     });
   });
@@ -408,9 +411,9 @@ describe('HypothesisApp', () => {
         const wrapper = createComponent();
         await clickLogOut(wrapper);
 
-        assert.calledOnce(fakeBridge.call);
+        assert.calledOnce(fakeFrameSync.notifyHost);
         assert.calledWithExactly(
-          fakeBridge.call,
+          fakeFrameSync.notifyHost,
           bridgeEvents.LOGOUT_REQUESTED
         );
       });
@@ -422,7 +425,7 @@ describe('HypothesisApp', () => {
         const wrapper = createComponent();
         await clickLogOut(wrapper);
 
-        assert.notCalled(fakeBridge.call);
+        assert.notCalled(fakeFrameSync.notifyHost);
       });
 
       it('does not call session.logout()', async () => {

src/sidebar/components/test/TopBar-test.js

@@ -9,7 +9,7 @@ import mockImportedComponents from '../../../test-util/mock-imported-components'
 
 describe('TopBar', () => {
   const fakeSettings = {};
-  let fakeBridge;
+  let fakeFrameSync;
   let fakeStore;
   let fakeStreamer;
   let fakeIsThirdPartyService;
@@ -26,8 +26,8 @@ describe('TopBar', () => {
       toggleSidebarPanel: sinon.stub(),
     };
 
-    fakeBridge = {
-      call: sinon.stub(),
+    fakeFrameSync = {
+      notifyHost: sinon.stub(),
     };
 
     fakeServiceConfig = sinon.stub().returns({});
@@ -60,7 +60,7 @@ describe('TopBar', () => {
     return mount(
       <TopBar
         auth={auth}
-        bridge={fakeBridge}
+        frameSync={fakeFrameSync}
         isSidebar={true}
         settings={fakeSettings}
         streamer={fakeStreamer}
@@ -123,7 +123,10 @@ describe('TopBar', () => {
           helpButton.props().onClick();
 
           assert.equal(fakeStore.toggleSidebarPanel.callCount, 0);
-          assert.calledWith(fakeBridge.call, bridgeEvents.HELP_REQUESTED);
+          assert.calledWith(
+            fakeFrameSync.notifyHost,
+            bridgeEvents.HELP_REQUESTED
+          );
         });
       });
     });

src/sidebar/components/test/UserMenu-test.js

@@ -9,7 +9,7 @@ import mockImportedComponents from '../../../test-util/mock-imported-components'
 
 describe('UserMenu', () => {
   let fakeAuth;
-  let fakeBridge;
+  let fakeFrameSync;
   let fakeIsThirdPartyUser;
   let fakeOnLogout;
   let fakeServiceConfig;
@@ -20,7 +20,7 @@ describe('UserMenu', () => {
     return mount(
       <UserMenu
         auth={fakeAuth}
-        bridge={fakeBridge}
+        frameSync={fakeFrameSync}
         onLogout={fakeOnLogout}
         settings={fakeSettings}
       />
@@ -40,7 +40,7 @@ describe('UserMenu', () => {
       userid: 'acct:eleanorFishtail@hypothes.is',
       username: 'eleanorFishy',
     };
-    fakeBridge = { call: sinon.stub() };
+    fakeFrameSync = { notifyHost: sinon.stub() };
     fakeIsThirdPartyUser = sinon.stub();
     fakeOnLogout = sinon.stub();
     fakeServiceConfig = sinon.stub();
@@ -147,8 +147,11 @@ describe('UserMenu', () => {
 
         onProfileSelected();
 
-        assert.equal(fakeBridge.call.callCount, 1);
-        assert.calledWith(fakeBridge.call, bridgeEvents.PROFILE_REQUESTED);
+        assert.equal(fakeFrameSync.notifyHost.callCount, 1);
+        assert.calledWith(
+          fakeFrameSync.notifyHost,
+          bridgeEvents.PROFILE_REQUESTED
+        );
       });
 
       it('should not fire profile event for first-party user', () => {
@@ -159,7 +162,7 @@ describe('UserMenu', () => {
 
         onProfileSelected();
 
-        assert.equal(fakeBridge.call.callCount, 0);
+        assert.equal(fakeFrameSync.notifyHost.callCount, 0);
       });
     });
   });
@@ -201,8 +204,8 @@ describe('UserMenu', () => {
 
         const openNotebookItem = findMenuItem(wrapper, 'Open notebook');
         openNotebookItem.props().onClick();
-        assert.calledOnce(fakeBridge.call);
-        assert.calledWith(fakeBridge.call, 'openNotebook', 'mygroup');
+        assert.calledOnce(fakeFrameSync.notifyHost);
+        assert.calledWith(fakeFrameSync.notifyHost, 'openNotebook', 'mygroup');
       });
 
       it('opens the notebook and closes itself when `n` is typed', () => {
@@ -215,8 +218,8 @@ describe('UserMenu', () => {
         assert.isTrue(wrapper.find('Menu').props().open);
 
         wrapper.find('.UserMenu').simulate('keydown', { key: 'n' });
-        assert.calledOnce(fakeBridge.call);
-        assert.calledWith(fakeBridge.call, 'openNotebook', 'mygroup');
+        assert.calledOnce(fakeFrameSync.notifyHost);
+        assert.calledWith(fakeFrameSync.notifyHost, 'openNotebook', 'mygroup');
         // Now the menu is "closed" again
         assert.isFalse(wrapper.find('Menu').props().open);
       });

src/sidebar/index.js

@@ -108,8 +108,6 @@ import LaunchErrorPanel from './components/LaunchErrorPanel';
 import { ServiceContext } from './service-context';
 
 // Services.
-import { Bridge } from '../shared/bridge';
-
 import { AnnotationsService } from './services/annotations';
 import { APIService } from './services/api';
 import { APIRoutesService } from './services/api-routes';
@@ -152,7 +150,6 @@ function startApp(config, appEl) {
     .register('apiRoutes', APIRoutesService)
     .register('auth', AuthService)
     .register('autosaveService', AutosaveService)
-    .register('bridge', Bridge)
     .register('features', FeaturesService)
     .register('frameSync', FrameSyncService)
     .register('groups', GroupsService)

src/sidebar/services/features.js

@@ -15,18 +15,18 @@ import { watch } from '../util/watch';
  */
 export class FeaturesService {
   /**
-   * @param {import('../../shared/bridge').Bridge} bridge
+   * @param {import('../services/frame-sync').FrameSyncService} frameSync
    * @param {import('../store').SidebarStore} store
    */
-  constructor(bridge, store) {
-    this._bridge = bridge;
+  constructor(frameSync, store) {
+    this._frameSync = frameSync;
     this._store = store;
   }
 
   init() {
     const currentFlags = () => this._store.profile().features;
     const sendFeatureFlags = () => {
-      this._bridge.call(
+      this._frameSync.notifyHost(
         bridgeEvents.FEATURE_FLAGS_UPDATED,
         currentFlags() || {}
       );

src/sidebar/services/frame-sync.js

 import debounce from 'lodash.debounce';
 
 import bridgeEvents from '../../shared/bridge-events';
+import { Bridge } from '../../shared/bridge';
 import { isReply, isPublic } from '../helpers/annotation-metadata';
 import { watch } from '../util/watch';
 
+/**
+ * @typedef {import('../../shared/port-rpc').PortRPC} PortRPC
+ */
+
 /**
  * Return a minimal representation of an annotation that can be sent from the
  * sidebar app to a connected frame.
@@ -47,11 +52,15 @@ export class FrameSyncService {
   /**
    * @param {Window} $window - Test seam
    * @param {import('./annotations').AnnotationsService} annotationsService
-   * @param {import('../../shared/bridge').Bridge} bridge
    * @param {import('../store').SidebarStore} store
    */
-  constructor($window, annotationsService, bridge, store) {
-    this._bridge = bridge;
+  constructor($window, annotationsService, store) {
+    /** Channel for sidebar <-> host communication. */
+    this._hostRPC = new Bridge();
+
+    /** Channel for sidebar <-> guest(s) communication. */
+    this._guestRPC = new Bridge();
+
     this._store = store;
     this._window = $window;
 
@@ -60,9 +69,9 @@ export class FrameSyncService {
 
     /**
      * Watch for changes to the set of annotations displayed in the sidebar and
-     * notify connected frames about new/updated/deleted annotations.
+     * notify connected guests about new/updated/deleted annotations.
      */
-    this._setupSyncToFrame = () => {
+    this._setupSyncToGuests = () => {
       let prevPublicAnns = 0;
 
       watch(
@@ -96,20 +105,20 @@ export class FrameSyncService {
           // when they are added or removed in the sidebar, but not re-anchoring
           // annotations if their selectors are updated.
           if (added.length > 0) {
-            bridge.call('loadAnnotations', added.map(formatAnnot));
+            this._guestRPC.call('loadAnnotations', added.map(formatAnnot));
             added.forEach(annot => {
               inFrame.add(annot.$tag);
             });
           }
           deleted.forEach(annot => {
-            bridge.call('deleteAnnotation', formatAnnot(annot));
+            this._guestRPC.call('deleteAnnotation', formatAnnot(annot));
             inFrame.delete(annot.$tag);
           });
 
           if (frames.length > 0) {
             if (frames.every(frame => frame.isAnnotationFetchComplete)) {
               if (publicAnns === 0 || publicAnns !== prevPublicAnns) {
-                bridge.call(
+                this._hostRPC.call(
                   bridgeEvents.PUBLIC_ANNOTATION_COUNT_CHANGED,
                   publicAnns
                 );
@@ -131,21 +140,21 @@ export class FrameSyncService {
     };
 
     /**
-     * Listen for messages coming in from connected frames and add new annotations
+     * Listen for messages coming in from connected guest frames and add new annotations
      * to the sidebar.
      */
-    this._setupSyncFromFrame = () => {
+    this._setupSyncFromGuests = () => {
       // A new annotation, note or highlight was created in the frame
-      bridge.on('beforeCreateAnnotation', event => {
+      this._guestRPC.on('beforeCreateAnnotation', event => {
         const annot = Object.assign({}, event.msg, { $tag: event.tag });
         // If user is not logged in, we can't really create a meaningful highlight
         // or annotation. Instead, we need to open the sidebar, show an error,
         // and delete the (unsaved) annotation so it gets un-selected in the
         // target document
         if (!store.isLoggedIn()) {
-          bridge.call('openSidebar');
+          this._hostRPC.call('openSidebar');
           store.openSidebarPanel('loginPrompt');
-          bridge.call('deleteAnnotation', formatAnnot(annot));
+          this._guestRPC.call('deleteAnnotation', formatAnnot(annot));
           return;
         }
         inFrame.add(event.tag);
@@ -154,7 +163,10 @@ export class FrameSyncService {
         annotationsService.create(annot);
       });
 
-      bridge.on('destroyFrame', frameIdentifier =>
+      // The `destroyFrame` message currently comes from the guests, but we'll
+      // likely need to route it via the host <-> sidebar channel to work around
+      // a Safari bug (https://bugs.webkit.org/show_bug.cgi?id=231167).
+      this._guestRPC.on('destroyFrame', frameIdentifier =>
         destroyFrame(frameIdentifier)
       );
 
@@ -172,7 +184,7 @@ export class FrameSyncService {
       }, 10);
 
       // Anchoring an annotation in the frame completed
-      bridge.on('sync', events_ => {
+      this._guestRPC.on('sync', events_ => {
         events_.forEach(event => {
           inFrame.add(event.tag);
           anchoringStatusUpdates[event.tag] = event.msg.$orphan
@@ -182,44 +194,38 @@ export class FrameSyncService {
         });
       });
 
-      bridge.on('showAnnotations', tags => {
+      this._guestRPC.on('showAnnotations', tags => {
         store.selectAnnotations(store.findIDsForTags(tags));
         store.selectTab('annotation');
       });
 
-      bridge.on('focusAnnotations', tags => {
+      this._guestRPC.on('focusAnnotations', tags => {
         store.focusAnnotations(tags || []);
       });
 
-      bridge.on('toggleAnnotationSelection', tags => {
+      this._guestRPC.on('toggleAnnotationSelection', tags => {
         store.toggleSelectedAnnotations(store.findIDsForTags(tags));
       });
 
-      bridge.on('sidebarOpened', () => {
-        store.setSidebarOpened(true);
+      this._guestRPC.on('openSidebar', () => {
+        this._hostRPC.call('openSidebar');
       });
 
-      // These invoke the matching methods by name on the Guests
-      bridge.on('openSidebar', () => {
-        bridge.call('openSidebar');
-      });
-      bridge.on('closeSidebar', () => {
-        bridge.call('closeSidebar');
-      });
-      bridge.on('setVisibleHighlights', state => {
-        bridge.call('setVisibleHighlights', state);
+      this._guestRPC.on('closeSidebar', () => {
+        this._hostRPC.call('closeSidebar');
       });
     };
   }
 
   /**
-   * Find and connect to Hypothesis clients in the current window.
+   * Connect to the host frame and guest frame(s) in the current browser tab.
    */
   connect() {
     /**
-     * Query the Hypothesis annotation client in a frame for the URL and metadata
-     * of the document that is currently loaded and add the result to the set of
-     * connected frames.
+     * Query the guest in a frame for the URL and metadata of the document that
+     * is currently loaded and add the result to the set of connected frames.
+     *
+     * @param {PortRPC} channel
      */
     const addFrame = channel => {
       channel.call('getDocumentInfo', (err, info) => {
@@ -236,13 +242,12 @@ export class FrameSyncService {
       });
     };
 
-    this._bridge.onConnect(addFrame);
+    this._guestRPC.onConnect(addFrame);
 
     // Listen for messages from new guest frames that want to connect.
     //
     // The message will include a `MessagePort` to use for communication with
-    // the guest. Communication with the host currently relies on the host
-    // frame also always being a guest frame.
+    // the guest.
     this._window.addEventListener('message', e => {
       if (e.data?.type !== 'hypothesisGuestReady') {
         return;
@@ -254,14 +259,40 @@ export class FrameSyncService {
         return;
       }
       const port = e.ports[0];
-      this._bridge.createChannel(port);
+      this._guestRPC.createChannel(port);
+    });
+
+    this._setupSyncToGuests();
+    this._setupSyncFromGuests();
+
+    this._hostRPC.on('sidebarOpened', () => {
+      this._store.setSidebarOpened(true);
     });
 
-    // Notify host frame that it is ready for guests to connect to it.
-    this._window.parent.postMessage({ type: 'hypothesisSidebarReady' }, '*');
+    // When user toggles the highlight visibility control in the sidebar container,
+    // update the visibility in all the guest frames.
+    this._hostRPC.on('setVisibleHighlights', state => {
+      this._guestRPC.call('setVisibleHighlights', state);
+    });
 
-    this._setupSyncToFrame();
-    this._setupSyncFromFrame();
+    // Create channel for sidebar <-> host communication and send port to host.
+    //
+    // This also serves to notify the host that the sidebar application is ready.
+    const hostChannel = new MessageChannel();
+    this._hostRPC.createChannel(hostChannel.port1);
+    this._window.parent.postMessage({ type: 'hypothesisSidebarReady' }, '*', [
+      hostChannel.port2,
+    ]);
+  }
+
+  /**
+   * Send an RPC message to the host frame.
+   *
+   * @param {string} method
+   * @param {any[]} args
+   */
+  notifyHost(method, ...args) {
+    this._hostRPC.call(method, ...args);
   }
 
   /**
@@ -274,7 +305,7 @@ export class FrameSyncService {
    */
   focusAnnotations(tags) {
     this._store.focusAnnotations(tags);
-    this._bridge.call('focusAnnotations', tags);
+    this._guestRPC.call('focusAnnotations', tags);
   }
 
   /**
@@ -283,6 +314,6 @@ export class FrameSyncService {
    * @param {string} tag
    */
   scrollToAnnotation(tag) {
-    this._bridge.call('scrollToAnnotation', tag);
+    this._guestRPC.call('scrollToAnnotation', tag);
   }
 }

src/sidebar/services/test/features-test.js

@@ -2,12 +2,12 @@ import bridgeEvents from '../../../shared/bridge-events';
 import { FeaturesService } from '../features';
 
 describe('FeaturesService', () => {
-  let fakeBridge;
+  let fakeFrameSync;
   let fakeStore;
 
   beforeEach(() => {
-    fakeBridge = {
-      call: sinon.stub(),
+    fakeFrameSync = {
+      notifyHost: sinon.stub(),
     };
 
     fakeStore = {
@@ -23,7 +23,7 @@ describe('FeaturesService', () => {
   });
 
   function createService() {
-    const service = new FeaturesService(fakeBridge, fakeStore);
+    const service = new FeaturesService(fakeFrameSync, fakeStore);
     service.init();
     return service;
   }
@@ -38,7 +38,7 @@ describe('FeaturesService', () => {
 
     // First update, with no changes to feature flags.
     notifyStoreSubscribers();
-    assert.notCalled(fakeBridge.call);
+    assert.notCalled(fakeFrameSync.notifyHost);
 
     // Second update, with changes to feature flags.
     fakeStore.profile.returns({
@@ -51,7 +51,7 @@ describe('FeaturesService', () => {
     notifyStoreSubscribers();
 
     assert.calledWith(
-      fakeBridge.call,
+      fakeFrameSync.notifyHost,
       bridgeEvents.FEATURE_FLAGS_UPDATED,
       fakeStore.profile().features
     );
@@ -62,7 +62,7 @@ describe('FeaturesService', () => {
 
     // First update, with no changes to frames.
     notifyStoreSubscribers();
-    assert.notCalled(fakeBridge.call);
+    assert.notCalled(fakeFrameSync.notifyHost);
 
     // Second update, with changes to frames.
     fakeStore.frames.returns([{ uri: 'https://example.com' }]);
@@ -70,7 +70,7 @@ describe('FeaturesService', () => {
     notifyStoreSubscribers();
 
     assert.calledWith(
-      fakeBridge.call,
+      fakeFrameSync.notifyHost,
       bridgeEvents.FEATURE_FLAGS_UPDATED,
       fakeStore.profile().features
     );